Multisig is a well-recognized idea for many in Bitcoin: a multisig transaction requires approval from a number of events earlier than it may be executed. We distinguish between “n-of-n” multi-signatures, the place the variety of concerned events is n, and so they all must approve, and “t-of-n” threshold signatures, the place solely a smaller quantity t of contributors must approve. Cryptographic schemes like MuSig, MuSig-DN and MuSig2 for multi-signatures and FROST by Komlo and Goldberg for threshold signatures can scale back transaction value and enhance privateness of multisig wallets.
Up to now, within the Bitcoin Group FROST has solely been utilized in experimental implementations. On this submit, we clarify why that is the case and the way we purpose to advance FROST in a Bitcoin manufacturing atmosphere by our current publication of a BIP draft for the ChillDKG distributed key technology protocol.
First, what are the advantages of FROST?
Privateness and Effectivity Beneficial properties with MuSig2 and FROST
With MuSig2 and FROST, regardless that a number of contributors contribute to the signing course of, the end result is a single signature.
This not solely offers higher privateness to the contributors by making the transaction appear to be as peculiar singlesig-wallet transaction. It additionally trims down the transaction, decreasing its dimension and due to this fact decreasing the transaction charge. All nice issues!
MuSig2 and FROST enable Bitcoin customers to function a multisig pockets with the identical transaction value as an everyday single-signature pockets. The fee advantages are particularly important for techniques with numerous signers and frequent transactions, akin to federated sidechains like Liquid or Fedimint. Not like conventional multisig, which leaves a definite fingerprint that permits blockchain observers to establish transactions of the pockets, FROST-based wallets are indistinguishable from common single-signature wallets on the blockchain. Due to this fact, they supply an enchancment in privateness in comparison with conventional multisig wallets.
Whereas MuSig2 has seen adoption from the Bitcoin trade, the identical can’t be stated for FROST so far as we all know. This can be shocking, contemplating the existence of a number of FROST implementations, akin to in ZF FROST (by the Zcash Basis), secp256kfun (by Lloyd Fournier), and an experimental implementation in libsecp256k1-zkp (by Jesse Posner and Blockstream Analysis). There may be even a IETF specification for FROST, RFC 9591 (although it’s not suitable with Bitcoin as a result of Taproot tweaking and x-only public keys). Some of the believable explanations is that FROST’s key technology course of is significantly extra complicated in comparison with MuSig2.
The Unresolved Puzzle of FROST in Manufacturing Programs
FROST primarily consists of two elements: key technology and signing. Whereas the signing course of carefully resembles that of MuSig2, key technology is considerably extra concerned than in MuSig2. Key technology in FROST is both trusted or distributed:
Trusted key technology entails a “trusted vendor” who generates the important thing and distributes key shares to the signers. The vendor represents a single level of failure: if malicious or hacked, the FROST pockets is vulnerable to being emptied.Distributed key technology (DKG), whereas eliminating the necessity for a trusted vendor, presents its personal challenges: All contributors have to be concerned in an interactive key technology “ceremony” run earlier than signing can begin.
The Core Problem: Settlement
DKG sometimes requires safe (i.e., authenticated and encrypted) channels between contributors to ship secret shares to particular person signers, and a safe settlement mechanism. The aim of the safe settlement mechanism is to make sure that all contributors finally attain settlement over the outcomes of the DKG, which embrace not solely parameters such because the generated threshold public key, but in addition whether or not no error occurred and the ceremony was not disrupted by a misbehaving participant.
Whereas the IETF specification considers DKG out of scope solely, the FROST implementations talked about above don’t implement safe settlement, leaving this activity to the library person. However settlement is just not trivial to implement: there exist numerous protocols and flavors of settlement, starting from easy echo broadcast schemes to full-fledged Byzantine consensus protocols, and their safety and availability ensures differ considerably, and generally subtly.
Regardless of the confusion which will come up as a result of this jungle of settlement protocols, the precise taste of settlement that DKG depends on is usually not clearly communicated to engineers, leaving them at nighttime.
ChillDKG: a Standalone DKG for FROST
To beat this impediment, we suggest ChillDKG, a brand new “ready-to-use” DKG protocol tailor-made to the use in FROST (draft). We offer an in depth description within the type of a draft of a Bitcoin Enchancment Proposal (BIP), which is meant to function a specification for implementers.
The primary characteristic of ChillDKG is that it’s standalone: The institution of safe communications and safe settlement is completed throughout the protocol, whereas all of this underlying complexity is hidden behind a easy and hard-to-misuse API. In consequence, ChillDKG is able to use in apply and doesn’t depend on any setup assumption, besides that every signer has selected the set of co-signers as recognized by particular person public keys. ChillDKG relies on the SimplPedPop protocol, in whose design and formal safety proof Blockstream Analysis has been concerned, see, the CRYPTO 2023 paper “Sensible Schnorr Threshold Signatures With out the Algebraic Group Mannequin” by Chu, Gerhart, Ruffing (Blockstream Analysis), and Schröder
Further targets for ChillDKG’s design embrace:
Broad applicability: ChillDKG helps a variety of situations, from these the place the signing gadgets are owned and related by a single particular person to these the place a number of homeowners handle the gadgets from distinct places.Easy backups: As an alternative of getting to again up secrets and techniques acquired from the opposite signers in a safe location, ChillDKG permits restoring the pockets solely from the system seed and public knowledge that’s the similar for all DKG contributors. Consequently, an attacker getting access to the general public backup knowledge doesn’t get hold of the key signing key, and if a person loses their backup, they’ll request it from one other trustworthy signer.
The ChillDKG BIP is at the moment in draft stage, and we’re looking for suggestions on design decisions and implementation particulars. Whereas the specification is generally full, it lacks take a look at vectors, and we’re contemplating including some extra options (e.g., “identifiable aborts”). As soon as finalized, the ChillDKG BIP can be utilized together with a BIP for FROST signing to instantiate your entire FROST protocol.
It is a visitor submit by Jonas Nick, Kiara Bickers, and Tim Ruffing. Opinions expressed are solely their very own and don’t essentially mirror these of BTC Inc or Bitcoin Journal.
